The invention relates to improved cathodic electrocoating vehicles containing blocked isocyanate functionality together with other functionality i.e. OH, NH, etc., capable of producing a self-cure in an electrocoating system.
In U.S. Pat. Nos. 3,984,299 and 4,031,050 Jerabek teaches a method of electrocoating wherein the electrodepositable (cathode) composition comprises a blocked polyisocyanate and a hydroxyl-containing adduct being the reaction product of a primary or secondary amine and a polyepoxide wherein the adduct is solubilized with acid to provide cationic groups. Hicks, U.S. Pat. No. 4,225,479 teaches aqueous resinous compositions salted with acid for electrodeposition wherein the composition is the reaction product of a polyepoxide and an amine mixture of C.sub.8-18 aliphatic monoamine and an aliphatic diamine containing one primary and one tertiary amine group reactive with epoxide groups. In U.S. Pat. No. 3,947,338, Jerabek and Marchetti teach cathodic electrocoating method using a self-curing polyurethane resin derived from the reaction product of an epoxide with a primary or secondary amine and a partially blocked organic polyisocyanate.
More recent systems relate to the copolymerization of various vinyl unsaturated monomers in the presence of epoxy resin adducts. Diefenbach et al, GB No. 3123536, teach an acidified aqueous binder for cathodic electrocoating wherein acrylic or methacrylic esters, hydroxy or amino substituted monomers and other non-functional vinyl monomers are copolymerized by emulsion polymerization in the presence of cationic synthetic resins such as adducts of epoxy resin with amines, poly caprolactone and diketimines. Subsequent mixing of a partially blocked diisocyanate further reacted with an alkane type polyol, i.e. trimethylol propane effects the crosslinking of the deposited coating. Gimpel et al, U.S. Pat. No. 4,399,256 have conducted polymerizations with copolymerizable N-(1-alkenyl)isocyanate, particularly vinyl isocyanate.
It is further known to free radical polymerize isocyanoethyl methacrylate (IEM) in dry solvents with itself or with a variety of acrylic or styrenic monomers without substantial damage to the isocyanate functionality (see Paul E. Cranely, A Latent Crosslinker for Coatings and Adhesive Resins, 27th Annual Technical Conference of the Cleveland Society for Coating Technology, May 15, 1984). Regulski and Thomas (Organic Coatings Applied Polymer Science Proc., 48, pp 1006 (1983)) determined deblocking temperatures for various blocked isocyanatoethyl methacrylates polymerized in the presence of methyl methacrylate and ethyl acrylate. Brixius and Simms, U.S. Pat. No. 4,446,175, teaches coatings based on IEM polymers and copolymers with various monomers using mercaptan chain-transfer agents. Bortnick (U.S. Pat. No. 2,718,516) had earlier described high molecular weight polymers based on (meth)acrylic ester isocyanates having a plurality of isocyanate groups. Oriel et al, U.S. Pat. No. 4,264,748 teach epoxy resin coating compositions cured with IEM/acrylate copolymers prepared from IEM or blocked IEM. Oriel and Flowers, U.S. Pat. No. 4,401,794 teach copolymerization of various, acrylate and vinyl aromatic monomers with isocyanatoalkyl esters of unsaturated carboxylic acid (IEM) under anhydrous conditions to form moisture curable coatings.
Hazan, U.S. Pat. No. 4,167,499 teaches a graft copolymer comprising a monoepoxide portion said to be grafted onto an acrylic-amine backbone having amine or hydroxyl functionality used in conjunction with conventional aminoplast crosslinkers. Sueling and Kuntz, U.S. Pat. No. 3,453,223, teach graft copolymerization of acrylonitrile, blocked polymerizable isocyanates under free radical catalysis in the presence of an aliphatically saturated alcohol having at least two primary or secondary hydroxyl groups. Schafer (U.S. Pat. No. 4,429,096) teaches copolymers prepared from acrylamide and the quaternary salt of para-isopropenyl-.alpha.,.alpha.-dimethylbenzyl isocyanate with diethylaminoalkanols or alkamines.
A major shortcoming of existing commercial cathodic electrocoat resin systems is the high (350.degree.-400.degree. F.) baking temperature required to achieve adequate coating cure and resistance properties. Where a cure temperature of less than 350.degree. F. is possible, there are usually problems such as lack of storage or tank stability.
The present invention relates to improved self-curing amino cation-active aqueous acid dispersible polymer coating compositions based on polymers and copolymers of unsaturated, polymerizable blocked isocyanate monomers, especially metaisopropenyl-.alpha.,.alpha.-dimethylbenzyl isocyanate.